Traditional Ziegler-Natta catalysts for olefin polymerization have dominated the landscape for decades, but a newer generation of single-site catalysts is gradually gaining acceptance. While single-site catalysts can provide polyolefins with superior physical properties, these advantages often come at the expense of reduced processability, high cost, or other tradeoffs. Thus, Ziegler-Natta catalysts remain important industry staples.
Indenoindolyl transition metal catalysts are single-site catalysts with remarkable versatility (see e.g., U.S. Pat. Nos. 6,232,260 and 6,908,972). The ligands are easy to synthesize and incorporate into complexes useful for making HDPE, LLDPE, plastomers, polypropylene, and other polyolefins. A continuing challenge with single-site catalysts generally and indenoindolyl catalysts in particular, however, is how to make resins that have both high molecular weight and a relatively low level of long-chain branching. Unbridged indenoindolyl catalysts provide ethylene polymers with the desired low level of long-chain branching, but the molecular weight is often lower than necessary for a particular application; a fractional melt index is difficult to obtain. In contrast, the bridged catalysts provide high enough molecular weight, but normally with considerable long-chain branching.
Recently, we observed that certain indenoindolyl complexes having “open architecture” (see U.S. Pat. Nos. 6,559,251 and 6,838,410) uniquely provide both high molecular weight and a low level of long-chain branching, and we wondered how best to take advantage of the attribute. Initial investigation of films made with these catalysts provided mixed results.
Resins from single-site catalysts are not commonly used for non-film applications such as pipe or blow-molded articles, and it is unclear how resin attributes such as long-chain branching might impact key properties for these applications. One commercial pipe resin, for instance, is a high-molecular-weight Ziegler-Natta resin, Equistar's L5008. This single resin provides pipe with good environmental stress crack resistance, but there is always room for improvement.
Bimodal high-density polyethylene grades having particular value for high-stiffness pipe applications are known (see J. Scheirs et al., “PE100 Resins for Pipe Applications: Continuing the Development into the 21st Century,” TRIP 4, December 1996, 408-415). The authors explain that commercial PE100 resins are HDPE grades “characterized by exceptionally high environmental stress crack resistance, good resistance to rapid crack propagation (RCP), and very high creep resistance.” The reference teaches the importance of concentrating short-chain branches in the high molecular weight fraction of the polymer chain distribution, and it further teaches tandem reactor systems or dual-site catalysts to achieve that effect. Not specifically taught, however, is how to select catalysts capable of achieving an acceptable PE100 resin. The reference is also silent regarding the importance of limiting the amount of long-chain branching in the resin.
U.S. Pat. No. 6,867,278 teaches a way to make a single resin useful for high-stiffness pipe. The resin is made in a slurry-loop reactor using a chromium catalyst on a fluoridated aluminophosphate support. The catalyst is believed to generate a high-molecular-weight ethylene copolymer having little or no long-chain branching (see col. 13). The '278 patent characterizes bimodal resins as having “excellent toughness” but “deficient when pipe is formed by extrusion under certain conditions” (see col. 1, II. 37-50).
Characterizing long-chain branching in polyolefins presents yet another challenge. Numerous methods have been proposed, including NMR and more complicated combinations of techniques, such as that proposed by Stadler et al. (Macromol. 39 (2006) 1474), which combines melt-state NMR, size-exclusion chromatography, multiangle laser light scattering, and linear viscoelastic shear rheology. Earlier, Shroff and Mpyridis recognized that a long-chain-branching index (“LCBI”) could be inferred from rheology data, including the measured intrinsic viscosity, a solution viscosity measurement, and the estimated zero-shear melt viscosity (Macromol. 34 (2001) 7362). Unfortunately, the known techniques are less useful for polymers with high molecular weight because evaluating the zero-shear viscosity (ηo) for such polymers becomes prohibitively difficult at high enough molecular weights.
In sum, the polyolefins industry would benefit from improved ways to characterize the amount of long-chain branching in olefin polymers, preferably without the need to devise a complex, new analytical method or instrument. Additionally, the industry needs resins and resin blends that can provide non-film articles such as pipe and blow-molded articles with enhanced properties. The industry would also benefit from finding ways to capitalize on the unique properties of resins made using indenoindolyl catalysts, particularly resins that have both high molecular weight and a limited content of long-chain branching.